Does Blender have a file locking feature?
No, for the same reasons ideasman42 said in his own answer. He suggests some workarounds however, here are mine..

Case A:
You can use PasteAll.org (which has natural support for blend files) to safely upload and password protect a .blend file, you can then send the link and password to the customer.

Case B:
Easy enough, they could work in separate personal files and merge them when ready. Link/Append

Ultimately, you are better off using an actual software that provides encryption such as 7zip.